Despite years of intensive study, it has only recently been realized that major cause of CD4 T cell loss inpatients infected with HIV is enhanced destruction of T cells, as opposed to reduced production. Numerous mechanisms have been proposed to account for the enhanced T cell destruction and likely, all of the proposed mechanisms contribute. Of interest, however, it remains controversial as to how cells directly infected with HIV die following infection. Numerous mechanisms have been proposed to account for infected cell killing, however none of the proposed mechanisms are universally accepted. It has long been recognized that one HIV protein, HIV protease, is intrinsically cytotoxic as its expression causes the death of bacteria, yeast, and mammalian cells, however it remains unclear how HIV protease kills these cells. Experimental data now demonstrate that HIV protease cleaves host cell proteins in addition to viral proteins. Our laboratory has recently defined one mechanism by which HIV protease can kill cells; specifically HIV3rotease can cleave the apoptosis initiating protein, procaspase 8, to result in its activation. Thereafter, a traditional apoptosis cascade is initiated, ultimately resulting in the structural, nuclear, and morphologic changes associated with apoptosis. While we have demonstrated that this mechanism of HIV protease mediated killing can occur, we have yet to determine whether or not this mechanism does occur, particularly in vivo. The focus of the current proposal is to define the relevance of this form of cell killing both in infections, which occur in the test tube, as well as infections of human lymphocytes. The clinical significance of this research may be reflected in recent clinical observations, suggesting an altered diseased course of patients who have infections with HIV that contain mutations within the HIV protease gene as compared to those that do not. We will further explore the potential impact of these mutations by assessing the differential impact of wild type HIV protease versus mutant protease on procaspase 8 activation and cell killing.